Framework of the Integrated Approach to Formation Mechanisms of Typical Combustion Byproducts─Polyhalogenated Dibenzo-p-dioxins/Dibenzofurans (PXDD/Fs)

Abstract

Understanding the mechanisms through which persistent organic pollutants (POPs) form during combustion processes is critical for controlling emissions of POPs, but the mechanisms through which most POPs form are poorly understood. Polyhalogenated dibenzo-p-dioxins and dibenzofurans (PXDD/Fs) are typical toxic POPs, and the formation mechanisms of PXDD/Fs are better understood than the mechanisms through which other POPs form. In this study, a framework for identifying detailed PXDD/Fs formation mechanisms was developed and reviewed. The latest laboratory studies in which organic free radical intermediates of PXDD/Fs have been detected in situ and isotope labeling methods have been used to trace transformation pathways were reviewed. These studies provided direct evidence for PXDD/Fs formation pathways. Quantum chemical calculations were performed to determine the rationality of proposed PXDD/Fs formation pathways involving different elementary reactions. Many field studies have been performed, and the PXDD/Fs congener patterns found were compared with PXDD/Fs congener patterns obtained in laboratory simulation studies and theoretical studies to mutually verify the dominant PXDD/Fs formation mechanisms. The integrated method involving laboratory simulation studies, theoretical calculations, and field studies described and reviewed here can be used to clarify the mechanisms involved in PXDD/Fs formation. This review brings together information about PXDD/Fs formation mechanisms and provides a methodological framework for investigating PXDD/Fs and other POPs formation mechanisms during combustion processes, which will help in the development of strategies for controlling POPs emissions.

Br-to-Cl Transformation Guided the Formation of Polyhalogenated Dibenzo-p-dioxins/Dibenzofurans

“…Furthermore, it is important to consider Cl–Br exchange reactions following the introduction of bromine, which have the potential to modify the congener patterns and lead to markedly more intricate fingerprints compared to those observed in single-halogen systems. , The increase in low-chlorinated CBz after the introduction of Br may be responsible for the increase in the fraction of PCDF congeners (such as 2,3,4,7,8-PeCDF and 2,3,4,6,7,8-HxCDF). PCDFs are the dominant PCDD/Fs formed by the precursor synthesis pathway from Cl-precursors, which can subsequently enhance the formation of PCDFs through the Br-to-Cl transformation pathway. , As shown in Figure a, the introduction of 1% Br increased the concentrations of 1,2-DiCBz and 1,2,3-TrCBz by 6.3 and 16.9%, respectively.…”

Formation of Pre-PCTA/DT Intermediates from 2-Chlorothiophenol on Silica Clusters: A Quantum Mechanical Study

“…The heterogeneous reactions in the post-flame and cool zone contribute to 70% of PCTA/DT formation [ 16 ]. Currently, the surface-mediated formation mechanism of PCTA/DTs can be divided into two categories: de novo synthesis in which residual carbon can react with halogen and sulfur elements on the surface of combustion-generated fly ash and precursor synthesis involving the catalytical coupling of chlorinated organic molecules [ 17 , 18 ]. Chlorothiophenols (CTPs) are important for the surface-mediated formation of PCTA/DTs because they are commonly demonstrated as crucial intermediates in de novo synthesis and predominant precursors in precursor synthesis [ 10 , 18 , 19 , 20 ].…”

“…The former refers to the reactions of surface-bound species with gaseous species, while the latter involves the coupling of two adsorbed surface species. Varieties of experimental research have proved that the L–H mechanism is responsible for the formation of PCDTs and the E–R mechanism is mainly related to the formation of PCTAs [ 17 , 22 , 23 , 24 , 25 ]. Nganai et al [ 25 ] have elucidated differences between the L–H and E–R mechanisms, which contribute to different rate orders during the oxidation of 2-chlorophenol on the CuO/Silica surface.…”

Enhancing Debromination Efficiency through Introducing Water Vapor Atmosphere to Overcome Limitations of Conventional Pyrolysis

“…However, due to the vast amount of brominated flame retardants contained in the WPCBs such as PBDEs and TBBA, 4 highly toxic bromine dioxins/furans may be released during smelting. 5,6 The high bromine content prevents WPCBs from recycling. Therefore, effective and advanced debromination treatments for WPCBs are necessary.…”

“…Waste printed circuit boards (WPCBs) are some of the core components of many e-waste systems, which have abundant valuable metals such as copper, lead, and gold, and these metals will bring great economic value if well recovered. However, due to the vast amount of brominated flame retardants contained in the WPCBs such as PBDEs and TBBA, highly toxic bromine dioxins/furans may be released during smelting. , The high bromine content prevents WPCBs from recycling. Therefore, effective and advanced debromination treatments for WPCBs are necessary.…”